An old piece of Will and Kate’s wedding fruitcake sold for $7,500. It’s a good reminder that fruitcake is gross.

An eight-tiered wedding cake, made by Fiona Cairns, inside Buckingham Palace in London shortly before Prince William married Kate Middleton in Westminster Abbey. (Reuters/Pool)

There’s someone in the world who paid $7,500 to buy an old piece of fruitcake.

No, that’s not a joke. Yes, it was bit of a publicity stunt. (How did you guess?)

The fruitcake in question is pretty special, I’ll give them that. It was served to guests at the wedding of Prince William and the duchess formerly known as Kate Middleton in April 2011.

The purchaser in question is Gee Chuang, chief executive of the Silicon Valley startup Listia, which plans to use the old wedding cake as a part of a company promotion on the royals’ fourth wedding anniversary.

The slightly sticky concoction of nuts, old dried fruit, (occasionally) alcohol, sugar and a bunch of other stuff might be good a few days after it is baked. But the whole point of fruitcake is that its proponents delight in the fact that you can eat it months, if not years, after it comes out of the oven.

Let that sink in. Years.

Science has even confirmed that you can probably eat certain types of fruitcake after the expiration date because, fruitcakes. Specifically, according to a new study, there’s so little moisture that very little — not even bacteria — can survive in there.

Fruitcakes might last forever.

“All of these dried and candied ingredients have what we call ‘low water activity’ — meaning they have very little moisture available,” Ben Chapman, a food safety researcher at North Carolina State University, said in a press release. “Low water activity is important because many microorganisms, including foodborne illness-causing bacteria, need moisture in order to reproduce.”

Contrary to popular belief, that’s not always a good thing. As Chapman notes, after all that time, “it might taste pretty bad.”

Look, I know plenty of people disagree. Among them are some very famous people, not the least of whom are the Duke and Duchess of Cambridge. There’s also Paul McCartney, who reportedly orders one every year from the same bakery used by the royals.

Fiona Cairns (R) and her team put the finishing touches on the royal wedding cake in April 2011. (Reuters/Pool)

Along with the piece of fruitcake, which has (hopefully) been frozen or refrigerated in its original presentation tin since the wedding, Chuang and Listia purchased a blank invitation to the event of the decade, printed on the queen’s stationery.

And as long as it is considered a novelty collectible item (and not an edible one), there’s nothing wrong with a little old wedding fruitcake.

Chuang said that up-bidding brought the sale price of the cake to $6,000 from the $1,000 to $2,000 it was originally projected to fetch. Add about $1,500 in fees and you’ve got one very expensive piece of old, gross food. Still, Chuang said it was less than his company had budgeted.

The fruitcake will be traded on the company’s listing marketplace next year, where you can “buy” it for free with Listia “currency” that can then be used to in trades on Listia’s marketplace.

Notably, Chuang doesn’t really like fruitcake either.

“Not particularly,” he said when asked by The Post if he has an appreciation for the dessert.

“Fruitcake is one of those things during the holidays you find yourself eating,” he said. “It’s a very festive thing, I think.”

He added: “Once a year, I do enjoy it, but it’s not one of those things you eat every month.”

So there you have it.

The best thing about fruitcake is still “The Tonight Show’s” Fruitcake Lady: